Three-way catalysts containing at least one noble metal selected from among Pt (platinum), Pd (palladium) and Rh (rhodium) are widely used to clean exhaust gases discharged from internal combustion engines such as automobile engines. In a typical constitution of such a three-way catalyst, a catalyst coat layer made of alumina is formed on the surface of a highly heat-resistant ceramic substrate, and one or two or more noble metals selected from among Pt, Pd and Rh are supported on the catalyst coat layer. Among these noble metals, Pd and Pt mainly contribute to carbon monoxide (CO) and hydrocarbon (HC) elimination performance, and Rh mainly contributes to NOx elimination performance (reductive elimination performance).
In order to efficiently eliminate components contained in exhaust gases using this type of three-way catalyst, it is preferable for the air/fuel ratio, which is the mixing ratio of air and gasoline supplied to the engine, to be close to the stoichiometric air-fuel ratio. In the past, OSC materials that were inorganic materials having oxygen storage capacity (OSC) were used as carriers for noble metals in order to alleviate fluctuations in air/fuel ratio atmospheres at which catalysts could operate effectively (for example, see PTL 1). An OSC material stores oxygen present in an exhaust gas when the air/fuel ratio of the exhaust gas is lean and converts the exhaust gas into a reducing atmosphere, thereby easily reducing NOx present in the exhaust gas. Meanwhile, when the air/fuel ratio of the exhaust gas becomes rich, oxygen stored by the OSC material is released and the exhaust gas is converted into an oxidizing atmosphere, thereby readily oxidizing CO and HC present in the exhaust gas. In this way, even if fluctuations occur in the oxygen concentration in the exhaust gas, stable catalytic performance can be achieved and the purifying performance of the catalyst is improved.